Position or object location systems are finding increasing application in manufacturing and materials handling environment. For example, such systems have utility for factory automation, including such application as tool automation, process control, robotics, computer integrated manufacturing (CIM), and just in time (JIT) inventory control.
One approach to position location systems is to use transmitters, commonly known as TAGs, that are attached to objects to be tracked by an array of receivers in a tracking area. Those TAGs have stored information that is transmitted to the receivers in the position location system by radio, ultrasonic or optical communications, using various techniques for identifying object movement or location in the near range of a receiver.
Radio communications provide a high degree of accuracy and performance in the terms of range per watt hour and penetrability through opaque structures. However, a problem with radio communications in a typical business environment, which includes walls, windows and other fixed structures, is that for the frequency of interest (i.e., above 100 MHz), random reflections introduce interference and path distortions in TAG transmissions arriving at a given receiver. Moreover, in such an environment, the unpredictable continuation of transmissions through walls and other structures make signal strength only marginally useful for communicating distance and location information.
Ultrasonic systems for locating personnel may include a portable battery power transceiver that is periodically scanned and which responds to signals from remote transceiver locations throughout the area of interest. Since the transceiver awaits scanning, power consumption is continuous. Utilizing the scan response transmission format may result in transceivers not transmitting because they are in a dead zone, i.e., transceivers out of range of the scanning transceiver. This inability to receive signals during a scan cycle will result in a "temporary lost personnel" status being transmitted to the personnel locator system. Additionally, since the distance from the transceiver unit to the remote transceiver location is variable, the transceiver design must accommodate worse case transmission ranges at the expense of additional power requirements.
Another problem that exists with ultrasonic personnel locating systems is due to their susceptibility to heavy ultrasonic noise. In most factory applications, there is a heavy ultrasonic background noise that interferes with the operation of the ultrasonic personnel locating systems. Additionally, ultrasonic signals are easily reflected by walls, cabinets, furniture, etc. These reflected signals provide interference signals. Since ultrasonic signals travel only at the speed of sound, the longer propagation delay of the reflected signals produce a more troublesome interference signal.
Optical systems for locating personnel may include the use of infrared transmitters to communicate information from the transmitter TAG to an infrared receiver. This type of an optical locator system has its TAGs powered by a small battery and the user of that TAG manually activates the TAG when that person is within the range of a receiver in the optical locator system. This type of system will reduce the size of the TAG's power requirement in transmitting information. However, this approach usually results in the personnel locator system reporting the "temporary lost personnel" for users who have simply forgotten to turn on their TAG when entering a room that has a TAG receiver.
Accordingly, a need exists for a position location system that incorporates portable transmitters that are low powered, light weight, easy to use and have utility in an environment subject to the effects of unwanted transmission.